1. Field of the Invention
The present invention relates to an apparatus for controlling steering angles of front and rear wheels of a vehicle.
2. Description of the Related Art
As a steering mode of a four-wheeler, a front wheel steering mode of giving steering angles only to front wheels is generally adopted. However, according to a front/rear wheel (four-wheel) steering mode of giving steering angles also to rear wheels, improvements of drivability and stability can be made, which include reduction of the minimum turning radius and stabilization of a cornering attitude at an intermediate high speed.
In the front/rear wheel steering mode, there is a mechanical transmission mode, in which steering systems of front and rear wheels are mechanically coupled to each other, and there is a mode realized in a steering angle control apparatus for front and rear wheels of a vehicle, in which the steering systems of the front and rear wheels are not mechanically coupled to the steering wheel, and adjustment can be made for steering angles of the front and rear wheels independently with respect to a manipulated variable of the steering wheel. In this apparatus, a method is conceived readily, which manipulates the steering wheel so that the steering angles of the front and rear wheels can be changed by an equal amount in a phase reverse to each other in order to reduce the minimum turning radius of the vehicle.
However, in such an apparatus, the vehicle has behaved such that a part thereof is greatly swung outward when the vehicle is parked parallel in a forward motion or when the vehicle is turned.
In order to solve the foregoing problem, the following method of suppressing a large outward swing of a part of the vehicle is conceived. Specifically, when steering angles of front and rear wheels of the vehicle are adjusted so as to realize target steering angles of the front and rear wheels, a reference steering angle in the case where a manipulated variable of a steering wheel is maintained constant is operated in response to the manipulated variable of the steering wheel, determination is made as to whether steering is in an increase state where a direction of steering wheel manipulation and a direction changed thereby coincide with each other or in a return state where the direction of steering wheel manipulation and a direction changed thereby contradict each other, and when the determination is made that the steering is in the increase state, the reference steering angle is corrected so that an azimuth xcex8 to a rotation center (hereinafter referred to as a xe2x80x9crotation center azimuthxe2x80x9d) can be reduced, and is set as the target steering angle, and when the determination is made that the steering is in the return state, the reference steering angle is corrected so that the rotation center azimuth xcex8 can be increased, and is set as the target steering angle.
However, since the direction changed by the manipulated variable of the steering wheel and a traveling direction of the vehicle are different from each other, in some cases, a feeling of wrongness that the traveling direction of the vehicle is difficult to be manipulated is given to a driver, or ride comfort of the driver and a passenger is deteriorated by a large lateral acceleration applied to the vehicle.
For example, such a problem is sometimes caused in parking the vehicle parallel in a forward motion. Now, description will be made for a point of this problem based on simulation results shown in FIGS. 19A to 24.
FIGS. 19A to 19E are time charts in the case of carrying out the parallel parking in the forward motion by steering only the front wheels, each chart showing a change as below. FIG. 19A is a chart of the manipulated variable of the steering wheel, FIG. 19B is a chart of the rotation center azimuth, FIG. 19C is a chart of the vehicle traveling direction, FIG. 19D is a chart of the variable of the vehicle traveling direction, and FIG. 19E is a chart of the change of the lateral acceleration. Here, when the manipulated variable of the steering wheel is a positive value, manipulation to the left direction with respect to the neutral position is shown, and when the manipulated variable is a negative value, manipulation to the right direction with respect to the neutral position is shown. Moreover, as shown in FIG. 20, the vehicle traveling direction is set at an angle formed by a traveling direction of a reference point on the vehicle and an axis of abscissas of FIG. 20, and a left-hand (counterclockwise) direction is defined as positive. Here, the reference point on the vehicle is set at a bisection point of a segment connecting a bisection point of a front axle and a bisection point of a rear axle, that is, a center of the four wheels. The lateral acceleration is defined as positive when it is applied to the right direction with respect to the front of the vehicle, and is defined as negative when it is applied to the left direction with respect thereto.
FIG. 20 is a view showing an orbit of the vehicle for each second in the simulation of the front wheel steering, which is shown in FIGS. 19A to 19E.
FIG. 21 is a view showing an orbit from time t1 to time t2 for each second in the orbit of the vehicle in the simulation of the front wheel steering, which is shown in FIGS. 19A to 19E.
Meanwhile, steering control for correcting the target steering angle is carried out so as to correct the rotation center azimuth xcex8 in response to the steering increase/steering return state, the steering control being carried out with respect to the manipulated variable of the steering wheel, which is equal to the simulation shown in FIGS. 19A to 19E. FIGS. 22A to 22E are diagrams showing the following respective values in this case. FIG. 22A is a diagram showing the manipulated variable of the steering wheel, the diagram being the same as the chart of FIG. 19A. FIG. 22B is a diagram showing a rotation center azimuth, FIG. 22C is a diagram showing a vehicle traveling direction, FIG. 22D is a diagram showing a variable of the vehicle traveling direction, and FIG. 22E is a chart showing a change of a lateral acceleration.
FIG. 23 is a diagram showing an orbit of the vehicle in the simulation of the front/rear wheel steering, which is shown in FIGS. 22A to 22E, for each second.
FIG. 24 is a diagram showing an orbit from the time t1 to the time t2 for each second in the orbit of the vehicle in the simulation of the front/rear wheel steering, which is shown in FIGS. 22A to 22E.
When a comparison is made between the vehicle traveling direction in the front wheel steering shown in FIG. 19C and the vehicle traveling direction in the front/rear wheel steering shown in FIG. 22C, it is understood that the vehicle traveling directions in the front wheel steering and the front/rear wheel steering are greatly different from each other in sections from the time t1 to the time t2 and from time t3 to time t4. Moreover, when a comparison is made between the vehicle orbit in the front wheel steering shown in FIG. 21 and the vehicle orbit of the front/rear wheel steering shown in FIG. 24, it is understood that, in the section from the time t1 to the time t2 while the manipulated variable of the steering wheel is changed to the right direction, the vehicle travels in the left direction in the front/rear wheel steering, whereas the vehicle travels in an approximately constant direction in the front wheel steering.
Hence, in some cases, the vehicle traveling direction does not coincide with the direction where the manipulated variable of the steering wheel is changed, due to the above-described phenomena, so that the feeling of wrongness that the vehicle traveling direction is difficult to be controlled is given to the driver.
Furthermore, when a comparison is made between FIG. 19D showing the variable of the vehicle traveling direction in the front wheel steering and FIG. 22D showing the variable of the vehicle traveling direction in the front/rear wheel steering, it is understood that very large changes are caused at the time t1 and the time t3 in the front/rear wheel steering. Moreover, when a comparison is made between FIG. 19E showing the lateral acceleration in the front wheel steering and FIG. 22E showing the lateral acceleration in the front/rear wheel steering, it is understood that very large lateral accelerations are applied to the vehicle at the time t1 and the time t3 in the front/rear wheel steering. Hence, the ride comfort of the driver and the passenger is sometimes deteriorated due to the above-described phenomena.
The present invention has an object to provide an apparatus for controlling steering angles of front and rear wheels of a vehicle, which is capable of decreasing the outward swing of the vehicle orbit during parallel-parking in the forward motion and vehicle turning while suppressing the feeling of wrongness that the traveling direction of a front/rear wheel steering vehicle is difficult to be controlled and the deterioration of the ride comfort due to the large lateral acceleration.
In order to achieve the foregoing object, a first aspect of the present invention is an apparatus for controlling steering angles of front and rear wheels of a vehicle, in which the steering angles of the front and rear wheels are controlled independently, the apparatus including: a target steering angle operation unit operating target steering angles of the front and rear wheels based on a manipulated variable of a steering wheel; a steering increase/steering return determination unit determining as to whether steering is in an increase state where a direction of steering wheel manipulation and a direction changed thereby coincide with each other or in a return state where the direction of steering wheel manipulation and the direction changed thereby contradict each other; a target rotation center azimuth operation unit operating a target rotation center azimuth so as to reduce a rotation center azimuth when the steering increase/steering return determination unit determines the steering increase state and operating the target rotation center azimuth so as to increase the rotation center azimuth when the steering increase/steering return determination unit determines the steering return state; a vehicle behavior estimation unit estimating a vehicle behavior based on the target rotation center azimuth; a corrected target rotation center azimuth operation unit operating a corrected target rotation center azimuth by limiting the target rotation center azimuth so that the estimated vehicle behavior cannot exceed a specified range; a corrected target steering angle operation unit operating a corrected target steering angle by correcting the target steering angle so as to realize the corrected target rotation center azimuth; and a steering angle adjustment unit adjusting each of the steering angles of the front and rear wheels of the vehicle so as to realize the corrected target steering angle.
According to the above-described constitution, the outward swing of the vehicle orbit during the parallel-parking in the forward motion and the vehicle turning can be decreased while preventing the feeling of wrongness that the traveling direction of the front/rear wheel steering vehicle is difficult to be controlled and the deterioration of the ride comfort.
Next, definitions of terms in the present invention will be clarified with reference to FIG. 13.
Reference Point on Vehicle (P)
An optional point fixed on a vehicle. Usually, a bisection point of a segment connecting a bisection point of a front axle and a bisection point of a rear axle is selected as the reference point. The center point of gravity of the vehicle may be selected as the reference point.
Vehicle Fixed Coordinate
A coordinate system fixed on a vehicle, in which an origin, an x-axis and a y-axis are defined. Hereinafter, as shown in FIG. 13, the reference point P on the vehicle is taken as the origin, the x-axis is set toward the front of the vehicle, and the y-axis is set toward the side of the vehicle. Here, with regard to the y-axis, a turning direction of the vehicle is defined as positive. In FIG. 13, since the vehicle turns to the right, the right side of the vehicle is defined as positive. When the vehicle turns to the left, the left side of the vehicle is defined as positive.
Attitude Angle
An angle xcex2 formed by a direction toward which the reference point P on the vehicle travels and the front of the vehicle (x-axis in FIG. 13), and a left-hand (counterclockwise) direction is defined as positive.
Steering Angle
Angles formed by the x-axis and respective wheels in FIG. 13. A steering angle of a front right wheel is denoted as xcex4fr, a steering angle of a front left wheel is denoted as xcex4fl, a steering angle of a rear left wheel is denoted as xcex4rl, and a steering angle of a rear right wheel is denoted as xcex4rr.
Rotation Center
A point on the vehicle fixed coordinate, which will be the center of turning when the vehicle turns with the steering angles of the front and rear wheels of the vehicle fixed constant.
Radius from Rotation Center
A distance R between the reference point P on the vehicle and the rotation center.
Rotation Center Azimuth
An angle formed by a line connecting the reference point P on the vehicle and the rotation center and a line extended from the reference point P on the vehicle to a lateral direction (parallel to the y-axis) of the vehicle. A rotation angle to the vehicle traveling direction is defined as positive. When the vehicle turns to the right, the counterclockwise rotation is positive. When the vehicle turns to the left, the clockwise rotation is positive.
Steering Increase
To change a manipulated variable of a steering wheel so that a turning radius of the vehicle can be reduced. Further steering to the right when turning the steering wheel to the right, and on the contrary, further steering to the left when turning the steering wheel to the left, are referred to as the steering increase.
Steering Return
To change the manipulated variable of steering wheel so that the turning radius of the vehicle is increased. Returning of the steering to the left when turning the steering wheel to the right, and on the contrary, returning of the steering to the right when turning the steering wheel to the left, are referred to as the steering return.